Transistors and other devices are connected together to form circuits, such as very large scale integrated circuits, ultra large scale integrated circuits, memory, and other types of circuits. When the size of transistors is reduced and device compaction is increased, problems may arise concerning parasitic capacitance, off state leakage, power consumption, and other characteristics of the device. Silicon on insulator (SOI) structures have been proposed in an attempt to overcome some of these problems. However, SOI structures have a high rate of defects, as it is difficult to produce thin, uniform semiconductor layers in fabrication. Defect problems in SOI structures include defects within a single wafer (e.g., the thickness of the wafer differs at various points on the wafer) and defects from wafer to wafer (e.g., an inconsistent mean Si layer thickness among SOI wafers).
Semiconductor devices include separate p-type and n-type regions. In each region, current conducts by majority carriers of a first conductive type. Minority carriers in the same region, which carry a charge of a conductive type that is opposite of the first conductive type, have a thermal equilibrium concentration that is much lower than that of majority carriers. In a p-type region, holes are majority carriers. In a n-type region, electrons are majority carriers. When a p-type region meets with a n-type region to form a pn junction, a depletion region is formed with a built in potential barrier that prevents the majority carrier of each side from crossing the pn junction. With a reverse bias applied to the two ends of the p and n regions, the potential barrier is further raised to prevent current flow via majority carriers. Minority carriers in each side of the pn junction can move across the junction freely and constitute reverse leakage currents, as they carry charges of a conductive type that is opposite of the first conductive type. Reverse leakage current is more pronounced in a narrow band gap semiconductor, because for the same majority carrier concentration, a narrow band gap semiconductor has a greater minority carrier concentration, and, hence a higher reverse leakage current. The pn junctions exist between the source/substrate regions and the drain/substrate regions (e.g., horizontally orientated) and between the source/channel, drain/channel regions (e.g., vertically orientated). By using a SOI structure, there are no source/substrate or drain/substrate regions, and the reverse leakage across the horizontal pn junction is eliminated. However, the source to channel, and the drain to channel leakage due to minority carriers crossing the vertical junction still exists, and, this channel leakage problem is worse for a narrower band gap semiconductor (e.g., in some embodiments, less than 1.1 eV) semiconductor.
As stated above, as devices are made smaller and smaller, channel length is generally reduced. Reductions in the channel length generally result in increased device speed, as gate delay typically decreases. However, a number of negative side effects may arise when channel length is reduced. Such negative side effects may include, among others, increased off-state leakage current due the threshold voltage roll-off (e.g., short channel effects).
Another way of increasing device speed is to use higher carrier mobility semiconductor materials to form the channel. Carrier mobility is generally a measure of the velocity at which charge carrier flows in a semiconductor under an external unit electric field. In a transistor device, carrier mobility is a measure of the velocity at which carriers (e.g., electrons and holes) flow through or across the device channel in the inversion layer. For example, higher carrier mobility has been found in narrow band gap materials that include Germanium (Ge). Germanium (Ge) has bulk electron and hole mobility of 3900 cm2/Vsec and 1900 cm2/Vsec, respectively, which are much higher that that of bulk electron and hole mobility of Silicon (Si), which are 1500 cm2/Vsec and 450 cm2/Vsec, respectively.
The band gap associated with a semiconductor material is generally based on the difference between the conduction band edge and the valence band edge. In general, a higher mobility semiconductor has narrower band gap. In Germanium, the band gap is approximately 0.67 eV, which is relatively small compared to that of Silicon (Si), which is approximately 1.1 eV.